Isodimorphic Crystallization and Tunable γ–α Phase Transition in Aliphatic Copolyamides: Critical Roles of Comonomer Defects and Conformational Evolution
Jianfei Xia, Shanshan Xu, Ying Zheng, Jian Zhou, Chengtao Yu, Guorong Shan, Yongzhong Bao, Pengju Pan
Abstract
Minor comonomer units can be included as defects in crystalline phase during the crystallization of isodimorphic copolyamides, further changing their interchain packing. However, a clear conclusion on how their crystalline structure and phase transition behavior are affected by the inclusion of comonomer defects is lacking. Herein, we systematically studied the polymorphic crystallization and Brill transition of aliphatic even–even random copolyamides, synthesized from nylon-6,6 salt and nylon-8,8 salt. All copolyamides exhibit isodimorphism and undergo an α–γ crystal transition (Brill transition) upon heating. Their Brill transition temperatures are composition-dependent and show a “pseudoeutectic” characteristic in the intermediate composition range. The copolyamides with pseudoeutectic compositions exhibit a unique irreversible Brill transition, as manifested by the remaining of the γ phase without transition upon cooling from melt. Intriguingly, the γ–α transition of pseudoeutectic copolyamides is crystallization temperature (Tc) dependent. The γ phase that crystallized at low Tc includes more minor comonomer units as defects, which raises the energy barrier for gauche–trans conformational transition, thus preventing the γ–α transition. The γ phase formed at high Tc has less minor comonomer units, facilitating the transformation into α phase due to easier activation of the gauche–trans transition. This work has elucidated the crucial role of comonomer defects on conformational evolution and Brill transition of polyamides.